Blind threaded fasteners are fasteners, either internally or externally threaded, that can be first installed into a hole in a panel with access to one side of the panel only, hence the term “blind”. Most of these fasteners contain three basic components: a head, an intermediate collapsible thin-walled counterbored portion of the shank, and an internally threaded region at the end. Typically the threaded area and a portion of the shank are first installed into the hole and then a compressive force is applied to the shank by pulling on the threaded region while supporting the head. This compression causes the shank to buckle outwardly, creating what is known as a “bulb” on the side of the sheet opposite the head. The threaded tool member used to compress the shank is then removed, leaving the fastener permanently gripped to the panel and restrained by the head in one direction and the bulb in the opposite direction.
Presently available tools for applying the compressive load (upset force) to collapse the shank can be divided into two broad categories, spin-spin and spin-pull types. Both types of tools utilize an anvil, also known as a nosepiece, from which the mandrel projects. Its function is to support the head of the fastener while the mandrel retracts or spins to force the insert toward the anvil. Spin-spin tools apply the compressive load to the shank by applying enough torque to a mandrel threaded into the fastener to produce the required load. Spin-spin tools are inexpensive, lightweight and simple to set up and use and are therefore generally preferred. However, because the upset force they can develop is limited, they generally cannot be used to install parts having thicker walls. Spin-pull tools spin the mating threaded member into position, then apply an axial pulling force by retracting the mandrel. The mandrel is then removed by spinning it in the opposite direction. Spin-pull tools are more complex and consequently more expensive and heavier than spin-spin tools. They are also more difficult to set up and require more maintenance. In spite of these disadvantages, spin-pull tools are the best available choice for thick-walled parts requiring higher compressive loads to form the bulb.
Spin-spin tools of the existing art are rather simple devices powered by a reversible motor, pneumatic or electric. A transmission of some sort is typically used to reduce speed and increase torque. The transmission output is connected to a threaded mandrel. The mandrel is supported axially by a thrust bearing and radially by a close fit in a nosepiece. The nosepiece supports the head of the insert when the compressive load is applied. Serrations are applied to the end of the nosepiece to prevent the insert from rotating when the mandrel is tightened.
A spin-spin tool operates as follows. An insert is first threaded onto the mandrel. This can be done either manually or the tool can be rotationally jogged in the forward direction. Regardless of whether the insert is threaded on manually or by using tool power, it must be installed until the head is just in contact with the serrated anvil. The next step is to install the body of the fastener into the hole in the panel into which it will be installed. It must be installed fully so that the underside of the head is in intimate contact with the outer surface of the panel. The tool is then activated in the forward direction and as the tool rotates, a compressive load is produced causing the fastener to form a bulb on the back side of the work piece until the tool stalls. The torque at which the tool stalls is controlled by a parameter that was adjusted during the set-up process. If the torque has been properly set it will induce enough load to properly form the bulb on the fastener but not be so great as to damage the fastener. To complete the cycle, the trigger is actuated in the reverse direction to unthread the mandrel from the installed insert.
Spin-pull tools of the existing art have two major components, those which produce the spinning action and those which produce the pulling action. The spinning action is provided by a reversible pneumatic or electric motor. Unless used to create the pulling force, motors are typically smaller than those used on spin-spin tools, because less torque is required.
Spin-pull tools operate as follows. First the tool mandrel is threaded into the insert. After the insert has been fitted to the mandrel, the body of the insert is installed into the hole in the panel into which it is to be installed. It must be installed fully so that the underside of the head is in intimate contact with the outer surface of the panel. After the insert is installed the pulling motion of the tool is initiated by the operator squeezing a trigger. Depending on the design of the tool the pulling motion continues until either a preset distance or a preset force has been achieved. The pulling motion will cause the fastener to bulb completely. After the pulling motion is complete, the mandrel spins in the reverse direction unthreading it from the fastener. Depending on the style of tool, this reverse rotation is either initiated by the operator squeezing a trigger or by the tool sensing the completion of the pulling motion. After the mandrel spins out of the installed insert, the tool is ready for the next cycle.
Both types of tools of the existing art use a conventional threaded mandrel to engage the threads of the insert. This member needs to be started, essentially meaning its helix needs to be aligned with the helix of the insert thread. This helix alignment is complicated by the fact that most internally threaded inserts have two separate helical grooves. One is the actual thread, which is the one that needs to be aligned with the mandrel thread. The second is of smaller size, is one half turn from the true thread, and is created at the minor diameter by the way in which the insert material flows during the form tapping operation. Many internally threaded blind inserts are form tapped for increased thread strength compared to cut tapping. If the mandrel thread starts into the smaller diameter helix at the minor diameter, it will bind after a very short amount of rotation. Even if the mandrel is started properly, time is required to thread the mandrel in and then unthread it at the end of the cycle. If one assumes a rotational speed of 1500 RPM and 10 threads engaged, the total in and out time is 0.8 seconds plus the time required for the motor to accelerate to rated speed. If an additional 0.2 seconds is estimated for the two accelerations, the total time becomes 1.0 second. For high volume applications, user expectations are currently for a 5.0 second total cycle time, in which case the non-productive thread in and thread out time is 20 percent of the total.
It is, therefore, an objective of this invention to provide an installation tool for blind-threaded inserts with force capability equal to or greater than that of existing spin-pull tools which makes it simpler to set up and use than existing spin-spin tools. It is a further objective of this invention to eliminate the problems of cross-threading and mandrel wear and to reduce the time required to engage and disengage the threaded member of the tool from the insert threads.